Vacuum Hose Solves Puzzle
The technician found the plugged EGR port, cleaned it out, and we thought it was fixed. Two weeks later the code returned.
I am sure many of you look at an exhaust gas recirculation (EGR) flow code and figure it is just another plugged EGR port.
I, too, fell victim to the same line of thinking until a few 1999 Mazda Miatas proved me wrong. This is a fairly common problem on Mazdas; there are technical service bulletins that explain the problem and how to clean the EGR port in the intake. I have not figured out what makes this model year different from the rest. However, I have found a way to fix them if cleaning the intake port doesn't do it.
The first one of these I encountered seemed like all of the rest. The technician found the plugged EGR port, cleaned it out, and we thought it was fixed. Two weeks later the code returned. Feeling he must not have removed all of the carbon, the technician removed the upper and lower intake chambers and cleaned them (even though he didn't see any carbon buildup at that time). A week later, the car returned with the same code. This time he replaced all the vacuum hoses that ran between the base of the EGR valve and the EGR boost sensor. There had been reports of the vacuum hoses and pipes getting plugged with carbon. Again the vehicle returned with the P0402 code; at that time, I was out of ideas. It was such a common problem I couldn't understand why we couldn't get it fixed.
I can't take credit for fixing the first one. The technician I was working with had obtained some information about a flaw in the upper intake and decided to try replacing it. A few weeks later, he was kind enough to call and tell me what he had done. It didn't make much sense to me at the time, but it did fix the problem.
Six months passed before the next 1999 Miata call with a P0402 code and this one had all of the same symptoms. This technician had found the technical service bulletin and had done all the EGR port cleaning he could. He had run new vacuum hoses and replaced the EGR valve but was still getting the P0402 code.
Since there had been some problems related to the intake on the first 1999 Miata, I wondered what would happen if we ran the EGR boost sensor hose to a different vacuum source on the intake manifold, instead of where it was originally attached to the port at the base of the EGR valve. If there was really a problem with the EGR passage, it should now set a code for low flow instead. Two weeks later, the technician called to thank me - the car had not been returned since he swapped the vacuum hose around.
Since that time, I have encountered more than a dozen of these EGR problems and have fixed them by swapping the vacuum hoses around. It turns out there is even a capped-off port on the intake to run the hose to, and then cap off the original port. It seems too easy!
|Guy Dibble is an Identifix Asian specialist. He is ASE master and L1 certified with additional certification as an Isuzu and Mazda master. He has 21 years of diagnostic repair experience.|
I suggested that the technician clean the MAF sensor elements and road test again. That was all it took! The transmission was no longer slipping, commanded line pressure was normal, and calculated load numbers were back to normal. Hard to believe a 10-cent repair was the ultimate fix for costly transmission failures!
This 2000 Chevrolet Suburban had a whole lot of slip goin' on. When it was first brought to the repair facility, they found brown fluid and a severe slip most of the time, especially at highway speeds with a steady throttle. The line pressure gauge showed 70 psi at highway speed and the torque converter clutch (TCC) slip - which should have been less than 50 rpm - read more than 300 rpm on the scan tool. Fourth gear would not hold with any load placed on the transmission.
The transmission was rebuilt and the vehicle was placed back in service without a follow-up line pressure test. A short time later, the vehicle was back at the shop, slipping again with brown fluid. A different rebuilt transmission was installed, and again the vehicle was sent out without a line pressure test. When it returned again with a slipping problem, a more thorough test was done. Line pressure was still 70 psi most of the time, with a maximum of 90 psi. It was also discovered that the power train control module (PCM) commanded the pressure control solenoid (PCS) to a low line pressure state of 0.9 to 1.0 amps at all times.
All pertinent PCM inputs such as throttle position, vehicle speed sensor, mass air flow (MAF) and engine coolant temperature checked normal, yet the PCS command would not budge from the 0.9 to 1.0 amp range. At this point, the technician called Identifix for some assistance.
Even in a full torque converter stall at 2000 rpm, the line pressure command stayed higher than expected, 0.5 amps. A typical PCS amperage value is closer to 0.7 amps at highway speed and 0 to 0.1 amps at wide open throttle. One bit of data that stood out: The "calculated load" was 2 percent at highway speed, and only 20 percent at wide open throttle.
The mass air flow (MAF) sensor is the primary input for calculated load. I had the technician disconnect the MAF sensor and road test. He was relieved when he called back and said the transmission was now shifting well and not slipping. I suggested that the technician clean the MAF sensor elements and road test again. That was all it took! The transmission was no longer slipping, commanded line pressure was normal, and calculated load numbers were back to normal. Hard to believe a 10-cent repair was the ultimate fix for costly transmission failures!
Note: There is a preliminary information bulletin for this condition but it only lists front-wheel drive transmissions. A generic information bulletin (No. 04-07-30-013A) lists the cause of this condition as an aftermarket, excessively over-oiled air filter (this vehicle still had the OEM air filter).
|Ralph Dahlen is an Identifix GM specialist. He is a certified GM master and an ASE master technician with L1 certification. He has been an automotive instructor for 21 years.|
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